System for accurate tunable diode lidar gas detection

Abstract

A system for accurate tunable diode lidar gas detection, in which an internal gas reference cell with a known concentration of the gas of interest, such as methane (CH4), is provided. The gas reference cell provides known absorptions at wavelengths particular to the gas. The beam from the tunable laser of the lidar system passes through the gas reference cell before scanning a scene, and provides a known level of gas absorption at those known absorption wavelengths. This guarantees there is a minimum signal to be detected by the lidar system, and provides a calibrated, known signal to measure. The temperature of the tunable laser may be controlled using a thermoelectric cooler, adjusting scans of the tunable laser with absorption from the gas reference cell. By making lidar measurements using such calibrated cells, better calibration the system and providing more accurate values gas concentration path length (CPL) from the scene.

What is claimed is: 
     
         1 . A lidar system for detecting a gas, comprising:
 a tunable laser,
 wherein wavelength for the tunable laser is modulated by changing a drive current, and the tunable laser has a wavelength range that is adjustable; 
   a gas reference cell,
 wherein the gas reference cell contains a predetermined concentration of a gas to be detected; and 
   one or more processors having access to a non-transitory computer readable storage medium loaded with computer instructions that, when executed on the one or more processors, implement operations comprising:
 directing output of the tunable laser to pass through the gas reference cell to one or more scattering points; 
 scanning the drive current so that the tunable laser produces laser light within the wavelength range, wherein points of time of the drive current scan have a predetermined relationship to drive current and to laser wavelength; 
 detecting returning laser light scattered from the one or more scattering points over a period of time; 
 analyzing signals derived from the detected returning laser light to detect a reduction in signal corresponding to at least one wavelength within the wavelength range; and 
 adjusting the wavelength range of the tunable laser so that the reduction in signal corresponds to a predetermined drive current within the drive current scan. 
   
     
     
         2 . The system of  claim 1 , wherein
 the at least one wavelength within the wavelength range that is corresponding to the reduction in signal also corresponds to an absorption wavelength for the gas contained in the gas reference cell.   
     
     
         3 . The system of  claim 1 , wherein
 the wavelength range of the tunable laser has a center position, and the predetermined drive current within the scan is within ±10% from the center position of the wavelength range.   
     
     
         4 . The system of  claim 3 , wherein,
 when the reduction in signal corresponding to at least one wavelength within the wavelength range corresponds to a drive current greater than ±10% from the center position,   the operations comprise adjusting the wavelength range of the tunable laser such that the reduction in signal corresponds to the predetermined drive current that is within ±10% from the center position.   
     
     
         5 . The system of  claim 1 , additionally comprising
 a thermoelectric cooler,   wherein temperature of the tunable laser is controlled electronically using the thermoelectric cooler, and   adjusting the wavelength range is controlled by adjusting the temperature of the tunable laser.   
     
     
         6 . The system of  claim 5 , wherein
 when the reduction in signal corresponding to at least one wavelength within the wavelength range corresponds to a lower drive current,   the operation of adjusting the temperature of the tunable laser represents an increase in temperature.   
     
     
         7 . The system of  claim 5 , wherein
 when the reduction in signal corresponding to at least one wavelength within the wavelength range corresponds to a higher drive current,   the operation of adjusting the temperature of the tunable laser represents a decrease in temperature.   
     
     
         8 . The system of  claim 1 , wherein
 the tunable laser is a distributed feedback diode laser.   
     
     
         9 . The system of  claim 1 , wherein
 the gas to be detected is methane (CH 4 ).   
     
     
         10 . The system of  claim 9 , wherein
 the gas reference cell additionally contains nitrogen gas (N 2 ), and the predetermined concentration in the gas reference cell of the gas to be detected is 5%.   
     
     
         11 . The system of  claim 10 , wherein
 the tunable laser is a distributed feedback diode laser designed to operate at a wavelength of 1651 nm.   
     
     
         12 . The system of  claim 1 , wherein
 the gas to be detected is selected from the group consisting of carbon dioxide (CO 2 ), ammonia (NH 3 ), ethylene (C 2 H 4 ), and hydrogen sulfide (H 2 S).   
     
     
         13 . The system of  claim 1 , wherein
 the gas reference cell has two windows, each with a center region, and with each of the windows having two non-parallel surfaces; and   the operation of directing output of the tunable laser to pass through the gas reference cell comprises directing the output through the center regions of the two windows.   
     
     
         14 . The system of  claim 13 , wherein
 the two windows are counter-beveled, with one of the windows at an angle of approximately 87 degrees from a line connecting the window centers and the other window at an angle of approximately −87 degrees from said line; and wherein   the two non-parallel surfaces of each of the two windows have a wedge angle greater than 0.2 degrees.   
     
     
         15 . The system of  claim 13 , wherein
 each of the two windows is coated with an anti-reflection coating.   
     
     
         16 . The system of  claim 15 , wherein
 the anti-reflection coating comprises magnesium fluoride (MgF 2 ).   
     
     
         17 . A method comprising:
 directing output of a tunable laser of a lidar system to pass through a gas reference cell to one or more scattering points, wherein the gas reference cell contains a predetermined concentration of a gas to be detected;   scanning a drive current of the tunable laser so that the tunable laser produces laser light within a wavelength range, wherein points of time of the drive current scan have a predetermined relationship to drive current and to laser wavelength;   detecting returning laser light scattered from the one or more scattering points over a period of time;   analyzing signals derived from the detected returning laser light to detect a reduction in signal corresponding to at least one wavelength within the wavelength range; and   adjusting the wavelength range of the tunable laser so that the reduction in signal corresponds to a predetermined drive current within the drive current scan.   
     
     
         18 . The method of  claim 17 , wherein
 the at least one wavelength within the wavelength range that is corresponding to the reduction in signal also corresponds to an absorption wavelength for the gas contained in the gas reference cell.   
     
     
         19 . The method of  claim 17 , wherein
 the wavelength range has a center position, and the predetermined drive current within the drive current scan is within ±10% from the center position of the wavelength range; and   when the reduction in signal corresponding to at least one wavelength within the wavelength range corresponds to a drive current greater than ±10% from the center position,   adjusting the wavelength range of the tunable laser.   
     
     
         20 . The method of  claim 17 , wherein
 the lidar system additionally comprises a thermoelectric cooler, and   wherein temperature of the tunable laser is controlled electronically using the thermoelectric cooler, and   adjusting the wavelength range is controlled by adjusting the temperature of the tunable laser.